Implement path:join

[krader1961]

I was working on adding unit tests for my fix for issue #1385 and realized that the test would benefit from a path:join command to accommodate the fact that on UNIX forward slash is used and on Windows backslash is used to separate path components. I worked around that shortcoming by using the equivalent Go function plus string concatenation to construct the test case. Nonetheless, it should be possible for Elvish programs to construct paths using native conventions for separating path components rather than assuming a forward slash works on every platform.